In recent years, metal construction materials have become recognized as practical and economical options for traditional lumber construction for residential and light commercial structures. Interest in metal construction materials has been elicited by drastically increased costs of lumber and by the significant losses of life and property that have occurred during earthquakes and other natural disasters in which shearing forces severely damaged wood framed structures, causing the buildings to collapse.
While new construction is subject to updated regulations which are designed to make the structures better able to withstand shear forces, this does not address the issue of countless older houses, apartment buildings and other buildings that are still in use. With many of the structures, retrofitting to enhance shear resistance can be as complex and expensive as rebuilding. Further, if this expense is undertaken, the reinforcement may add strength to the wall only, and not the overall structure as is necessary to prevent collapse under intense, repeated shear conditions.
Conventional means for shear reinforcement consist of "diaphragming" the frame structure. This process involves removal of both existing interior and exterior surfaces and attachment of a solid sheet of material on each side, typically plywood in wood frame construction, to wall studs, nailing the plywood to the wall studs around the plywood sheet's perimeter at spacings dictated by well-known engineering standards. This method relies upon the structure's frame alone for support, and provides no means for enhancing shear resistance by attaching the shear panel independently to the structure's foundation. This provides limited shear resistance which, during the repeated exposure to shearing forces such as might occur during aftershocks of a major earthquake, stresses the nails and plywood to their own material limits, shearing off the nails and splintering the plywood. Since the shear panels are not attached to the foundation, there is no resistance to uplift or lateral motion of the entire structure. Further, the specified shear resistance requires modification of large areas of the wall to attach a sufficient amount of plywood to provide the required reinforcement, which can be expensive and highly disruptive considering the fact that both the exterior and interior surfaces must be removed and replaced.
Methods have been proposed for creating an I-beam-like structure with the heads of the beam abutting the wall studs and the web of the beam spanning the space between the studs. However, this has similar disadvantages to the plywood panel except that the web buckles or folds under shear pressure instead of splintering. Further, there is an added issue of uninterrupted thermal conductivity across the entire panel, which has a negative impact on the structure's insulation.
An alternative means for adding shear resistance is X-bracing, in which two steel beams are attached diagonally from the foundation to the truss upper joist. These are applied to the exterior of the wall only, which, nonetheless, can be highly disruptive, interfering with the building's weather resistant qualities, and requiring removal and replacement of detailed decorative exterior finishes.
A lumber-compatible lightweight metal construction system has been disclosed in grandparent application Ser. No. 08/190,643 of Bass, which has been allowed, which is incorporated in its entirety by reference. This metal construction system is based upon a triangle-cross-sectioned beam with flanges extending from the triangle's apex. The beam's openable configuration allows it to be easily connected to other beams and other construction materials, including framing lumber and paneling. A significant advantage of this metal construction system is that adjacent lengths of triangular beams are connected only at relatively small portions of the beams' overall lengths, so that thermal conduction is minimized from one beam to another. This represents a significant improvement over prior art metal construction systems, which suffered from, among other things, problems relating to the inability to effectively insulate walls framed with metal beams to compete with the energy efficiency of wood-framed construction. The metal construction system of Bass further permitted placement of insulation within the spacing to construct structures with insulation "R" values rivaling those for wood frame construction.
It would be desirable to provide an apparatus for providing improved shear resistance which can be incorporated both in new and existing structures, which apparatus can be easily constructed and installed on the interior sides of the structure's walls with minimal disruption; it would also be desirable to provide a method for retrofitting existing structures to increase shear resistance.